Conventional keyboards generally include a high profile, full-travel keyswitches that included a plunger that is slidably mounted within a housing. The plunger is designed to slide downwardly in a linear path when the keytop is depressed. Downward pressure on the keytop actuates an electrical switch beneath the plunger.
In many applications, including alphanumeric keyboards for notebook and lap top computers, it is desirable to have keyboards with keyswitches having a shorter overall height, generally referred to as "low profile" keyswitches. One problem which has been encountered in designing low profile keyswitches is that it has been difficult to incorporate desirable characteristics of the prior high profile keyswitches while reducing the keyswitch height. Such desirable characteristics included full-travel so that the amount of downward movement of the keytop would be substantially the same in the "low profile" design as in the "full profile" design. Other favorable features included a tactile field and overtravel. In the past low profile keyboards have been associated with low quality keyswitches that were undesirable for high speed data entry.
A full-travel keyboard generally refers to a substantial movement of the keytop so that an operator will only activate a keyswitch through intentional movement of the keytop over a desired depression distance. Generally a "full-travel" keyboard has keyswitches that require downward movement of approximately four millimeters in travel. A "low profile" keyboard generally refers to the height of the keyswitch assembly from the top of the keytop to the lower part of the keyswitch structure. Preferably it should be approximately eight millimeters or less in total height.
A further desirable feature is that of overtravel in which the electrical contact is made before the keytop is fully depressed. An over-travel feature provides for more reliable switch operation because keyboard operators often inadvertently fail to press the keyswitch down the entire distance, particularly when typing at high rates of speed. When an over-travel feature is provided, the keyswitch will remain activated or "turn on" as long as the plunger is depressed at least to the point where electrical contact is made. The structure provides that the key may be pressed beyond the electrical contact point while maintaining the switch in a closed condition.
It is particularly desirable in notebook and lap top personal computers to minimize the size and weight of the entire computer. The size and weight of the keyboard is very important since it is generally built in or intimately attached to the housing of the personal computer. However the use of low profile keyboards in notebook and lap top personal computers give the impression that such computers are inferior for high speed data entry.
It is further desirable to minimize the number of keyboard components to reduce the costs of manufacture and assembly.
One suggested way to reduce the number of components is to use unitary molded rubber keyboards such as illustrated in the Calder U.S. Pat. No. 4,540,865 granted Sep. 10, 1985 and the Church U.S. Pat. No. 4,764,770 granted Aug. 16, 1988. Molded rubber keyboards have been used in combination with overlying printed circuit boards to provide a keyboard structure which is less expensive than those keyboards having full travel and a separate push button member separately mounted in a frame. The molded rubber keyboards generally comprise a elastomeric sheet integral with upwardly projecting rubber keys. Each elastomeric key is located above a recess in the sheet and has a depending projection to actuate the keyswitches on the printed circuit board located directly below.
A principal disadvantage, besides not being able to provide a full-travel keyboard, is a tendency of each individual keytop of the unitary rubber key structure to wobble or move laterally as it is depressed. The user feels that the keyswitch lacks precision in operation and thus identifies the keyboard as being of a low quality.
To overcome such disadvantages, U.S. Pat. No. 4,540,865 provides guides 38 intermediate the keytops to maintain the keytops substantially level as the keytop is depressed. U.S. Pat. No. 4,764,770 provides an interconnecting membrane 40 for maintaining the keytops in a linear path as the keytops are depressed.
One of the objects of this invention is to provide a keyboard that overcomes the above-identified problems in providing a full-travel keyboard having tactile feel and overtravel features utilizing an integral key structure.
These and other objectives and advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.